Silicon-based modulators have received considerable attention in recent years. In silicon, the modulation mechanism is dominated by free-carrier plasma dispersion effect (FCPD). The FCPD effect occurs when a variation in the free carrier concentration causes a corresponding change in refractive index and optical extinction coefficient or absorption coefficient, leading to phase-shift. Free carrier concentration can be varied either by using p-i-n diodes, or metal-oxide-semiconductor (MOS) capacitors based device structures.
Pin offers high effective refractive index variation and hence high phase modulation efficiency. However, speed rarely exceeds 1 GHz due to its long carrier recombination process. The MOS modulation scheme, in contrast, can offer high speed and zero DC power. However, in MOS accumulation the carriers are concentrated near the gate dielectric in very thin region, e.g., about a 10 nm thin layer. Thus the free carrier optical overlap is very small, resulting in a relatively low effective real refractive index change for the similar electrical field applied or power. For a π phase shift to occur as required to achieve signal modulation, a long phase shifter length will then be needed.
In relation to variation of free carrier concentration using p-i-n diodes, an approach is disclosed in publication “Micrometer-scale silicon electro-optic modulator”, Qianfan Xu et al., Nature, Vol. 435, pp. 325-327, 2005. The publication discloses a silicon electro-optic modulator. The modulator consists of a ring resonator coupled to a single waveguide. The transmission of the waveguide is highly sensitive to the signal wavelength and is greatly reduced at wavelengths in which the ring circumference corresponds to an integer number of guided wavelengths. By tuning the effective index of the ring resonator, the resonance wavelength is modified, which induces a strong modulation of the transmitted signal. Thus the effective index of the ring resonator is modulated electrically by injecting electrons and holes using a p-i-n junction embedded in the ring resonator.
Another similar approach using p-i-n diodes is disclosed in publication “High-speed silicon electrooptic modulator design”, Fuwan Gan et al., IEEE Photonics Technology Letters, Vol. 17, No. 5, May 2005, 1007-1009. The publication discloses a high-speed electronic carrier-injection modulator based on a high-index-contrast split-ridge waveguide and integrated p-i-n-diode section. The split-ridge waveguide includes a high-index ridge separated from a high-index slab via a thin low index layer, thereby combining the advantages of a buried waveguide and a ridge waveguide. The index layer which splits the ridge wave-guide is rather thin, so that there is a good heat sinking to the slab portion of the waveguide acting as a heat spreader. The optical mode is well confined within the ridge in horizontal direction, which greatly reduces loss due to highly doped contact regions and sidewall roughness.
Yet another similar approach using p-i-n diode is disclosed in publication “Optical Phase Modulators for MHz and GHz Modulation in Silicon-On-Insulator (SOI)”, Ching Eng Png et al., Journal of Lightwave Technology, Vol. 22, No. 6, June 2004. Publication discloses a low-loss single-mode optical phase modulator based on silicon-on-insulator (SOI) material. The modulator operates by injecting free carriers to change the refractive index in the guiding region. The overlap between the injected free carriers in the intrinsic region and the propagating optical mode has been optimized and a particular p-i-n device geometry where two n+ regions are joined as a common cathode has been employed.
In relation to variation of free carrier concentration using MOS capacitors, an approach is disclosed in U.S. Pat. No. 6,845,198. The patent discloses a silicon-based electro-optic modulator based on forming a gate region of a first conductivity to partially overlap a body region of a second conductivity type, with a relatively thin dielectric layer interposed between the contiguous portions of the gate and body regions. The modulator may be formed on an SOI platform, with the body region formed in the relatively thin silicon surface layer of the SOI structure and the gate region formed of a relatively thin silicon layer overlying the SOI structure. The doping in the gate and body regions is controlled to form lightly doped regions above and below the dielectric, thus defining the active region of the device. The optical electric field essentially coincides with the free carrier concentration area in the active device region. The application of a modulation signal causes simultaneous accumulation, depletion or inversion of free carriers on both sides of the dielectric at the same time, resulting in high speed operation.
A similar approach using MOS capacitors is disclosed in publication “Scaling the Modulation Bandwidth and Phase Efficiency of a Silicon Optical Modulator”, Ansheng Liu et al., IEEE Journal of selected topics in quantum electronics., Vol. 11, No. 2, March/April 2005. The publication discloses an all-silicon optical modulator based on a silicon waveguide phase shifter containing a MOS capacitor. The publication discloses that shrinking the waveguide size and reducing gate oxide thickness significantly enhances the phase modulation efficiency because of the optical field enhancement in the voltage induced charge layers of the MOS capacitor, which, in turn, induces refractive index change, and thus phase change in silicon due to free carrier dispersion effects.
Another approach using MOS capacitors is disclosed in publication “Phase Modulation Efficiency and Transmission Loss of Silicon Optical Phase Shifters”, Ling Liao et al., IEEE Journal of Quantum Electronics, Vol. 41, No. 2, February 2005. The publication focuses on understanding phase modulation efficiency and optical loss of MOS-capacitors-based silicon waveguide phase shifters. In the publication, a total of nine designs have been fabricated using poly-silicon and characterized at wavelengths around 1.55 μm. Detailed comparison of design parameters shows that scaling down the waveguide dimensions, placing the capacitor gate oxide near the center of the optical mode, and reducing the oxide thickness significantly enhances phase modulation efficiency.
Yet another approach using MOS capacitors is disclosed in publication “A High-performance Si-based MOS Electrooptic Phase Modulator With a Shunt-Capacitor Configuration”, Xiaoguang Tu et al., Journal of Lightwave Technology, Vol. 24, No. 2, February 2006. The publication proposes enhancing the optical overlap by introducing two dielectric layers and employing a high-confinement waveguide design. The VπLπ figure-of-merit achieved is approximately 1.0 Vcm in order to achieve 180 degree or π-shift of its phase change under 1V applied on a 1 cm long device. The publication discloses a novel Si-based MOS electro-optic phase modulator including two shunt oxide layer capacitors integrated on a SOI waveguide. The refractive index near the two gate oxide layers is modified by the free carrier dispersion effect induced by applying a positive bias on the electrodes. The theoretical calculation of free carrier distribution coupled with optical guided mode propagation characteristics has been carried out and the influence of the structure parameters such as the width and the doping level of the active region are analyzed.
Publication “Compact gate-all-around silicon light modulator for ultra high speed operation”, K. E. Moselund et al., Sensors and Actuators A130-131, pp. 220-227, 2006 also attempted to enhance the optical overlap by proposing a Gate-All-Round (GAA) sub-micrometer-scale waveguide and achieved a VπLπ of ˜0.45 Vcm. The publication discloses a GAA MOS capacitor consisting of a silicon nano-wire (Si-NW) surrounded by a thin gate oxide and a polysilicon gate. The light propagates in the entire GAA structure and not only the Si-NW.
Yet a further approach using MOS capacitors is disclosed in publication “Modeling and analysis of high-speed electro-optic modulation in high confinement silicon waveguides using metal-oxide-semiconductor configuration”, C. A. Barrios et al., Journal of Applied Physics, Vol. 96, No. 11, December, 2004. The publication analyzes the electrical and optical properties of a silicon electro-optic waveguide modulator using a MOS configuration. The device performance is studied under different modes of operation of the MOS diode and gate oxide thickness. The waveguide modulator structure consist of a high aspect ratio rib SOI waveguide with highly doped regions defined in the slab at each side of the rib. A poly-silicon layer acts as a gate electrode, whereas the lateral highly doped regions operate as ground electrodes. A top SiO2 cladding layer covers the whole structure.